Reduced glare intraocular lens

ABSTRACT

An intraocular lens for reducing aberrant optical effects includes an anterior surface, a posterior surface and a peripheral region/zone disposed about a central optical axis. The peripheral region/zone has an inflection region/transition area that is inclined with respect to the anterior surface at an angle between about 40 degrees and 120 degrees with respect to the optical axis.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Application No. 61/666,413,filed on Jun. 29, 2012, the contents of which are incorporated herein byreference for all purposes. Full Paris Convention priority is herebyexpressly reserved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application relates generally to intraocular lenses, and morespecifically to intraocular lenses with reduced aberrant opticaleffects, such as reduced positive and/or negative dysphotopsia.

2. Description of the Related Art

A human eye can suffer diseases that impair a patient's vision. Forinstance, a cataract may increase the opacity of the natural crystallinelens, causing blindness. To restore the patient's vision, the opaquelens may be surgically removed and replaced with an artificialintraocular lens, or IOL. An IOL may also be implanted to treatpresbyopia or for other elective ocular surgical procedures. The IOL canbe an accommodating IOL, which can adjust its axial position and/orshape to vary the optical power within a range in response to muscleaction in the eye. As a result, the patient can focus on objects in arange of distances from the eye, rather than at one discrete distance.The IOL may also be a multifocal IOL utilizing a refractive and/ordiffractive surfaces resulting in multiple focal points.

Healthy phakic eyes typically have a non-compromised visual field ofabout 60 degrees in the nasal direction, 105 degrees in the temporaldirection, 65 degrees in the superior direction, and 70 degrees in theinferior direction. With current IOLs, pseudophakic eyes may havereduced the field of view.

In addition, undesirable optical effects can arise after implantation ofan IOL. One of the undesirable optical effects is dysphotopsia which isdefined as the appearance of unwanted visible patterns. It is believedthat light refracted into the IOL can reflect from a sharp or truncatededge of the IOL thereby causing glare, positive dysphotopsia, or otheraberrant optical effects. Positive dysphotopsia can refer to theappearance of bright optical artifacts such as rings, halos, arcs orstreaks. Negative dysphotopsia can refer to the appearance of darkshadows or lines in the field of vision. Negative dysphotopsia may occurwhen some light rays that enter the eye and are either (1) not incidenton the IOL and pass by the IOL or (2) incident on the IOL edge, whileimmediately adjacent light rays enter the IOL and are refracted by andpass through the IOL onto the retina. Thus, IOLs that can reduce ormitigate aberrant optical effects, such as positive and/or negativedysphotopsia, as well as increase field of view are desirable.

SUMMARY OF THE INVENTION

The systems, methods and devices of the disclosure each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein.

Embodiments disclosed herein are directed to devices and methods forproviding corrective vision in the event the natural lens is replace. Insome embodiments, it would be desirable to have an artificial IOL thatcan reduce or mitigate dysphotopsia, or other aberrant optical effectsand regain the phakic field of view.

In one aspect, an IOL is provided that can reduce or mitigatedysphotopsia. In IOLs, one of the causes of dysphotopsia is theinteraction of light that is refracted by the IOL with the edge of theIOL. Accordingly, a possible solution to reduce or mitigate dysphotopsiais to design an IOL such that the edge of the IOL is outside the path oflight rays entering the eye and incident on the IOL. In such a designsince light rays incident on the edge of the IOL is minimized oreliminated, dysphotopsia can be reduce or eliminated. In variousimplementations, the IOL has an anterior surface and a posterior surfacethat are intersected by an optical axis. The anterior and posteriorsurfaces are joined by a peripheral region. Peripheral light from theside and behind a patient's eye enters the cornea refracting at amaximum angle of about r₁ degrees. These rays are incident on theanterior surface of the IOL and are refracted at a maximum angle of r₂degrees. For a refractive surface, the peripheral region is inclined atan angle of inclination greater than

${\sin^{- 1}\left( {\frac{n_{2}}{n_{3}}\sin \mspace{11mu} r_{1}} \right)},$

where n₃ is the refractive index of the material of the intraocularlens, n₂ is the combined refractive index of the cornea and the aqueoushumor. When the anterior and/or posterior surface contains a diffractivesurface, formulas based on diffractive optics are applicable as known tothose skilled in the art. The angle of inclination in this sense isdefined with respect to an axis parallel to the central optical axis OA,intersecting the peripheral region at the inflection point and extendingin the posterior direction from the inflection point. Additionally, theperipheral region may angle posteriorly from the anterior surface. Byway of example, in a 20 Diopter IOL, the peripheral region may beinclined posteriorly and defined at an angle greater than about 40degrees in order to prevent rays from striking the edge of the IOL.

In one preferred embodiment, an intraocular lens is comprised of ananterior optical surface extending peripherally from a central opticalaxis of the intraocular lens; a posterior optical surface extendingperipherally from the central optical axis; and a peripheral zonedisposed about and extending laterally from the anterior opticalsurface, the peripheral zone being inclined posteriorly from theanterior optical surface; wherein the extent of the posterior incline ofthe peripheral zone is sufficient to prevent aberrant optical effectsfrom high angle optical rays directed posteriorly toward the intraocularlens and refracted by the anterior surface. The peripheral zone maycomprise of a peripheral surface extending laterally and posteriorlyfrom a point of inflection disposed between the anterior surface and theperipheral zone. The point of inflection may be disposed laterally fromthe central optical axis by a distance greater than the distance to thelocation of the optic where the rays of greatest divergence refractedinto the eye by the cornea strike the anterior surface of the lens whenimplanted in the capsular bag of a patient's eye. The point ofinflection may be disposed laterally of the optical axis by at leastabout 2 mm, and is preferably at least about 2.5 mm, but may beconfigured to match the capsular bag size which is typically up to atleast about 5 mm. An angle may be provided between the peripheralsurface and an axis extending posteriorly from the point of inflectiondisposed between the anterior surface and the peripheral zone, whereinthe angle is greater than or equal to a maximum angle of refraction bythe anterior surface of the rays of greatest divergence refracted intothe eye by the cornea. The aforementioned angle may be greater than orequal to about 40 degrees and is preferably greater than or equal toabout 55 degrees, and more preferably greater than or equal to about 60degrees. Depending on the configuration, the angle may be as large asabout 120 degrees. For a generally circular IOL embodiment, the diameterof the anterior surface of the lens as viewed from a top plan view is inthe range of about 4 mm-9 mm and is preferably between about 7-9 mm ormore preferably between about 8-9 mm. For a generally circular IOLembodiment, the diameter of the posterior surface of the lens as viewedfrom a top plan view is in the range of about 5 mm-11 mm and ispreferably between about 8-11 mm or more preferably between about 9-11mm.

In another preferred embodiment, the intraocular lens may be comprisedof an anterior optical surface extending peripherally from a centraloptical axis of the intraocular lens; a posterior optical surfaceextending peripherally from the central optical axis; and a peripheralsurface disposed about and extending laterally from the anterior opticalsurface, the peripheral surface being inclined posteriorly from theanterior optical surface; wherein the intraocular lens is configured tominimize dysphotopsia by preventing peripheral light rays from passingthrough the peripheral surface of the lens. The intraocular lens may beconfigured to minimize negative and/or positive dysphotopsia with theperipheral surface located laterally outward of the trajectory ofperipheral light rays refracted by the anterior surface of the lens.

In another preferred embodiment, a dysphotopsia reducing intraocularlens may be comprised of an optic configured for implantation in the eyeof a patient, the optic having anterior surface and posterior surfacesintersected by an optical axis, the anterior and posterior surfacesbeing joined by a transition area disposed about the optical axis,wherein the transition area inclines posteriorly from the anteriorsurface and intersects the anterior surface at an angle greater thanapproximately 40 degrees with respect to the optical axis. The rays ofgreatest divergence refracted into the eye by the cornea strike theanterior surface of the lens when implanted in the capsular bag of apatient's eye at the intersection of the first edge and the anteriorsurface. The rays of greatest divergence refracted into the eye by thecornea may be refracted by the anterior surface such that they are notincident on the first edge.

In another preferred embodiment, an intraocular lens may be comprised ofan optic configured for implantation in the eye of a patient, the optichaving anterior surface and posterior surfaces intersected by an opticalaxis, the anterior and posterior surfaces being joined by a peripheralregion, the peripheral region inclined posteriorly from the anteriorsurface, the angle of inclination of the peripheral region being greaterthan

${\sin^{- 1}\left( {\frac{n_{2}}{n_{3}}\sin \mspace{11mu} r_{1}} \right)},$

where n₃ is the refractive index of the material of the intraocularlens, n₂ is the refractive index of aqueous humor and r₁ is the angle ofrefraction at which the most peripheral rays are refracted into the eyeby the cornea.

Details of one or more implementations of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings, and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments disclosed herein may be better understood from the followingdetailed description when read in conjunction with the accompanyingdrawings. Such embodiments, which are for illustrative purposes only,depict novel and non-obvious aspects of the inventions. The drawingsinclude the following figures.

FIG. 1 is a schematic representation of certain aspects of a human eyewith an artificial IOL positioned therein configured such that the mostperipheral rays that enter the eye are incident on and anterior opticalsurface of the IOL and not incident on a peripheral region, such as anedge of the IOL.

FIG. 2 is a schematic perspective view of an implementation of the IOLdepicted in FIG. 1, showing a central optical axis.

FIG. 3 is a cross-sectional view of another implementation of an IOL inwhich the most peripheral rays that enter the eye and are incident onand refracted by the IOL are not incident on an edge of the IOL.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A human eye includes a transparent crystalline biconvex lens which canfocus light from objects over a wide range of distances on the retina.The natural lens allows the eye to focus on the objects at variousdistances by changing its shape thereby changing its focal length. Theability of the lens to change its shape to adjust the focal length isknown as accommodation. The lens is housed in a structure known as thecapsular bag 102. During natural accommodation, the capsular bag isacted on by a ciliary muscle and zonular fibers (also known as zonules)in the eye, which can pull on the capsular bag to change its shape. Themotion of the capsular bag generally deforms the lens in order to changeits power, so that the eye can focus on objects at varying distancesaway from the eye.

In a healthy human eye ambient light is refracted into the eye by thecornea 101 and focused by the lens on the retina to form an image. Theimage is produced by the combination of the optical powers of the cornea101, the capsular bag 102 and the lens, all of which are generallydisposed about a central optical axis OA. As used herein, an “anteriordirection” is in the direction generally toward the cornea, while a“posterior direction” is generally in the direction toward the retinawhich is located rearward of the cornea 101.

In a healthy human eye, an iris is disposed between the cornea 101 andthe capsular bag 102 which provides a variable pupil that dilates underlower lighting conditions (scotopic vision) and constricts underbrighter lighting conditions (photopic vision) to control the amount ofambient light that enters the eye.

The average diameter of the cornea in a human eye is between about 10 mmand 12 mm. The radius of curvature of the cornea is typically betweenabout 6 mm and about 11.5 mm. The average distance between the mid-pointof the cornea and the capsular bag is between about 2.0 mm and 5.0 mm.In general, the average horizontal diameter of the natural lens isbetween 9-10 mm and the average thickness of the natural lens is about4.5 mm. The pupil diameter can vary between about 1.0 mm and about 8 mm.

FIG. 1 illustrates a cross-sectional view of a human eye in which an IOL103 is implanted in the capsular bag 102 to replace the natural lens.FIG. 2 is a schematic perspective view of the implementation of the IOL103 illustrated in FIG. 1. Although, the IOL 103 is illustrated as beingimplanted in the evacuated capsular bag 102, it is understood by aperson having ordinary skill in the art that the IOL 103 can be aphakic/piggy-back IOL which acts as a secondary lens in a phakic eyethat includes the natural lens. Also, it will be understood that the IOL103 may have haptics (not shown) to mechanically position the optic inposition in the eye. The IOL 103 has an anterior surface 201 and aposterior surface 203 that is intersected by the central optical axisOA. In use, the optical axis OA may extend from the fovea of the retinato an object being viewed. The IOL 103 also includes a peripheral region205 that is disposed between the anterior surface 201 and the posteriorsurface 203. The peripheral region 205 can join the anterior surface 201and the posterior surface 203. In various implementations, theperipheral region 205 may comprise a circular, elliptical or otherregular shaped peripheral zone that extends posteriorly from theanterior surface 201.

The figures suggest that a very precise demarcation can be providedbetween discrete regions of the IOL 103, such as between the anteriorzone 201 and the peripheral region 205. However, in some embodiments, agradual transition can be provided between these and other zones. Forexample, in various implementations, the peripheral region 205 caninclude an inflection region 207 (illustrated in FIG. 3) that forms atransition area between the anterior surface and the peripheral region205. The inflection region 207 may be inclined posteriorly with respectto the anterior surface 201 as discussed above. In variousimplementations, the inflection region can include a peripheral surfacewhich connects the anterior surface 201 to the peripheral region 205.

The IOL 103 is generally made of a transparent bio-compatible materialthat can be deformed. For example, in various implementations, the IOL103 can be made of silicone or acrylic. The anterior and/or theposterior surface of the IOL 103 are curved such that the IOL 103 hasoptical power. The anterior and/or posterior surface may also becomprised of a diffractive surface. Or, the lens may be moveable withrespect to the retina or other surface or deform to have adjustablepower, as in an accommodating IOL.

The field-of-view of an average human eye is about 110 degrees in thehorizontal direction. Accordingly, the most peripheral rays of light areincident on the cornea 101 at a maximum angle i₁ of about 110 degreeswith respect to the central optical axis OA, as illustrated by ray 105,and are refracted by the cornea 101 into the eye, as illustrated by ray107. Peripheral rays that are incident on the cornea 101 at an anglegreater than about 110 degrees with respect to the central optical axisOA will not enter the eye, which is the reason for the limitedfield-of-view of the human eye. Rather, these rays will be reflected byor pass through the opposite side of the cornea. If the geometry of thecornea at the incident point of ray 105 and the refractive index of thecornea 101 and aqueous humor are known, the angle of refraction r₁ ofthe refracted ray of light 107 can be determined from Snell's law ofrefraction. Mathematically, Snell's law of refraction is expressed as

${\frac{\sin \mspace{11mu} i}{\sin \mspace{11mu} r} = \frac{n_{2}}{n_{1}}},$

where i is the angle of incidence of a ray of light that is incidentfrom a medium having a refractive index n₁ onto a medium havingrefractive index n₂ and r is the angle of refraction. With reference toFIG. 1, n₁ is the refractive index of air which is considered to be 1.0and n₂ is the combined refractive index of the cornea and the aqueoushumor which is about 1.38. For a typical human eye, the most peripheralrays (e.g. ray 105) that are incident at an angle of about 110 degreeswith respect to the central optical axis OA are refracted by the cornea101 into the eye with an angle of about 80 degrees. In other words, fora typical human eye, r₁ is about 80 degrees.

The most peripheral rays that are refracted into the eye by the cornea(e.g. ray 107) are incident on the anterior surface 201 of the IOL 103and refracted into IOL 103 in accordance with Snell's law of refraction,as illustrated by ray 109. The angle r₂ that ray 109 makes with respectto an axis parallel to the central optical axis OA, intersecting theperipheral region at the inflection point and extending in the posteriordirection from the inflection point can be calculated from Snell's lawof refraction if the geometry of the IOL 103 at the incidence point ofray 107 and the refractive index of the material of the IOL 103 isknown. For the implementation illustrated in FIG. 1, the angle r₂ isgiven by

${\sin^{- 1}\left( {\frac{n_{2}}{n_{3}}\sin \mspace{11mu} r_{1}} \right)},$

where n₃ is the refractive index of the material of the IOL 103.Generally, for an acrylic or silicone IOL with a low refractive index,the angle r₂ is less than or equal to about 40 degrees for a typicalhuman eye having r₁ of about 80 degrees.

In the embodiment of FIG. 1, the IOL 103 is configured such that theperipheral region 205 is disposed laterally of the point of incidence ofthe ray 107 with the anterior surface 201 of the IOL 103. Additionally,the peripheral region 205 is disposed away from the trajectory of therefracted ray 109. In other words, the ray 109 may be refracted by theIOL 103 along a path therethrough but the path does not intersect theperipheral region 205. In one embodiment, the region 205 may be inclinedposteriorly from the anterior surface 201 and is at an angle θ greaterthan or equal to about 40 degrees and is preferably greater than orequal to about 55 degrees, and more preferably greater than or equal toabout 60 degrees. The peripheral region may be substantially straightthus maintaining this angle. Or if the peripheral region is comprised ofa curved portion, it may be configured such that light rays will notstrike the peripheral region of the IOL. In other words, the mostperipheral rays that enter the eye and are refracted into the IOL 103would not be incident on peripheral region 205 and also would not berefracted by the IOL to pass through the peripheral region 205. Thus,the interaction between the light that is refracted into the IOL 103 andthe peripheral region 205 can be reduced or eliminated which can preventaberrant optical effects such as positive and/or negative dysphotopsia.Since, the angle of inclination θ of the peripheral region 205 dependson the phenomenon of refraction, in various implementations of the IOL103, the angle of inclination θ of the peripheral region 205 isdetermined by the refractive index of the material of the IOL 103 andthe geometry of the portion of the anterior surface 103 at which themost peripheral rays that enter the eye are incident. For a typicalsilicone or acrylic IOL, the angle θ may be in the range of about 40degrees and 120 degrees, and is preferably in the range of about 40degrees and 60 degrees, and more preferably in the range of about 55degrees and 60 degrees.

As discussed above, one of the causes for negative dysphotopsia in someIOL designs is the creation of a shadow in the eye. The shadow can be ina region of the retina between two groups of rays that are incident onthe retina. The first group of rays pass laterally of the IOL are notrefracted at all by the lens. The second group of rays, which areimmediately adjacent to the first group, are incident on the lens andare refracted at an angle away from the first group. This causes the twogroups of rays to diverge, with little or no light being present in theregion between the diverging rays. Thus, the region between thediverging rays is darker, i.e., a shadow is cast on the retina. The IOL103 is configured such that the inflection region or the transition area207 that is inclined posteriorly from the anterior surface 201 isdisposed at a distance L from the central optical axis OA. If angle θ isgreater than 90 degrees, then at least a portion of the inflectionregion or the transition area is inclined anteriorly. The distance L isselected to be equal to or greater than the outermost point of incidenceof the ray 107. This ensures that the most peripheral rays that enterthe eye are incident on the anterior surface and not on the peripheralzone 205. In such implementations, negative dysphotopsia can also bereduced or mitigated since all light that enters the eye is incident onthe anterior surface of the IOL 103 and refracted in the preferred way.In various implementations, the inflection region can be disposed at adistance L of about 2-5 mm from the central optical axis OA.

Although FIGS. 1 and 2 illustrate the IOL 103 to be polygonal in shape,a person having ordinary skill in the art would understand that theanterior surface 201, the posterior surface 203 and the peripheralregion 205 can be curved to produce the desired power. In variousimplementations, the anterior surface 201 or the inflection region canhave some curvature. In those implementations, where the peripheralregion 205 is arcuate, the angle of the inclination of the peripheralregion 205 can be taken as the angle between an anterior-posterior lineparallel to the central optical axis and a line connecting a point ofinflection of the peripheral region 205 closer to the anterior surfaceand a point located at the boundary between the peripheral zone 205 andthe posterior surface 203. In some implementations, where the peripheralregion 205 is arcuate, the angle of inclination of the peripheral region205 can be taken as the angle between the largest chord of theperipheral region 205 and an axis that is parallel to the centraloptical axis and extends posteriorly from the point of inflection.

In various implementations, the IOL 103 can be designed by selectingparameters such as the lateral distance of the peripheral region 205from the central optical axis, the curvature of the peripheral region205, the angle of inclination of the peripheral region 205 such that themost peripheral rays that enter an average human eye are incident on theanterior surface of the IOL 103 and do not intersect the peripheralregion 205 after being refracted by the IOL 103. In someimplementations, the IOL 103 can be designed specifically for apatient's eye by taking the patient's pupil diameter, depth of thecapsular bag from a mid-point of the cornea into consideration such thatmost peripheral rays that enter the patient's eye are incident on theanterior surface of the IOL and do not intersect the peripheral region205 after being refracted by the IOL. In other implementations, a set ofIOLs designed for different pupil diameters and different depth of thecapsular bag from a mid-point of the cornea can be provided to suit theneeds of the general population.

The above described design considerations can also be used to designimplementations or contact lenses, spectacles or other ophthalmologicvisual aid devices to avoid aberrant optical effects.

The description of the invention and its applications as set forthherein is illustrative and is not intended to limit the scope of theinvention. Variations and modifications of the embodiments disclosedherein are possible, and practical alternatives to and equivalents ofthe various elements of the embodiments would be understood to those ofordinary skill in the art upon study of this patent document. These andother variations and modifications of the embodiments disclosed hereinmay be made without departing from the scope and spirit of theinvention.

What is claimed is:
 1. A intraocular lens comprising: an anterioroptical surface extending peripherally from a central optical axis ofthe intraocular lens; a posterior optical surface extending peripherallyfrom the central optical axis; and a peripheral zone disposed about andextending laterally from the anterior optical surface, the peripheralzone being inclined posteriorly from the anterior optical surface;wherein the extent of the posterior incline of the peripheral zone issufficient to prevent aberrant optical effects from high angle opticalrays directed posteriorly toward the intraocular lens and refracted bythe anterior surface.
 2. The intraocular lens of claim 1, wherein theperipheral zone comprises a peripheral surface extending laterally andposteriorly from a point of inflection disposed between the anteriorsurface and the peripheral zone.
 3. The intraocular lens of claim 2,wherein the point of inflection is disposed laterally from the centraloptical axis by a distance greater than the distance to the location ofthe optic where the rays of greatest divergence refracted into the eyeby the cornea strike the anterior surface of the lens when implanted inthe capsular bag of a patient's eye.
 4. The intraocular lens of claim 3,wherein an angle is provided between the peripheral surface and an axisextending posteriorly from the point of inflection disposed between theanterior surface and the peripheral zone, the angle being greater thanor equal to a maximum angle of refraction by the anterior surface of therays of greatest divergence refracted into the eye by the cornea.
 5. Theintraocular lens of claim 4, wherein the angle is greater than or equalto about 40 degrees.
 6. The intraocular lens of claim 4, wherein theangle is greater than or equal to about 55 degrees.
 7. The intraocularlens of claim 4, wherein the angle is greater than or equal to about 60degrees.
 8. The intraocular lens of claim 4, wherein the point ofinflection is disposed laterally of the optical axis by at least about 2mm.
 9. A intraocular lens comprising: an anterior optical surfaceextending peripherally from a central optical axis of the intraocularlens; a posterior optical surface extending peripherally from thecentral optical axis; and a peripheral surface disposed about andextending laterally from the anterior optical surface, the peripheralsurface being inclined posteriorly from the anterior optical surface;wherein the intraocular lens is configured to minimize dysphotopsia bypreventing light rays from passing through the peripheral surface of thelens.
 10. The intraocular lens of claim 9, wherein the intraocular lensis configured to minimize negative dysphotopsia.
 11. The intraocularlens of claim 9, wherein the intraocular lens is configured to minimizepositive dysphotopsia.
 12. The intraocular lens of claim 9, wherein theperipheral surface is located laterally outward of the trajectory oflight rays refracted by the anterior surface of the lens.
 13. Adysphotopsia reducing intraocular lens comprising: an optic configuredfor implantation in the eye of a patient, the optic having anteriorsurface and posterior surfaces intersected by an optical axis, theanterior and posterior surfaces being joined by a transition areadisposed about the optical axis, wherein the transition area inclinesposteriorly from the anterior surface and intersects the anteriorsurface at an angle greater than approximately 55 degrees with respectto the optical axis.
 14. The intraocular lens of claim 13, wherein raysof greatest divergence refracted into the eye by the cornea strike theanterior surface of the lens when implanted in the capsular bag of apatient's eye at the intersection of the first edge and the anteriorsurface.
 15. The intraocular lens of claim 14, wherein rays of greatestdivergence refracted into the eye by the cornea is refracted by theanterior surface such that they are not incident on the first edge. 16.An intraocular lens comprising: an optic configured for implantation inthe eye of a patient, the optic having anterior surface and posteriorsurfaces intersected by an optical axis, the anterior and posteriorsurfaces being joined by a peripheral region, the peripheral regioninclined posteriorly from the anterior surface, the angle of inclinationof the peripheral region being greater than${\sin^{- 1}\left( {\frac{n_{2}}{n_{3}}\sin \mspace{11mu} r_{1}} \right)},$where n₃ is the refractive index of the material of the intraocularlens, n₂ is the refractive index of aqueous humor and r₁ is the angle ofrefraction at which the most peripheral rays are refracted into the eyeby the cornea.